Documentation of Control.Exception should take better account of imprecise exceptions

The documentation for throwIO currently says

Although throwIO has a type that is an instance of the type of throw, the two functions are subtly different:

throw e   `seq` x  ===> throw e
throwIO e `seq` x  ===> x

The first example will cause the exception e to be raised, whereas the second one won't.

But throw e `seq` x ===> throw e will only hold if x doesn't diverge itself due to imprecise exception semnatics, so that reduction is plain wrong.

We should also very explicitly document the fact that throw throws an imprecise exception, whereas throwIO throws a precise exception and give a short example of imprecise exception semantics. Currently, the distinction is not very prominent at all in the haddocks and a bit scattered between the module haddock and evaluate. There's no clear line that says "It is undefined whether error "foo" seq error "bar" throws foo or bar", or generally that a `seq` b === b `seq` a. Also it would help to point to pseq as a pure alternative to evaluate.